Plasters and cements composed of gypsum have been used as building materials for thousands of years and are still used for walls, floors, surface finishes, sculptures and industrial manufacturing. Raw gypsum is heated to drive out molecular traces of water and then ground into a powder known as plaster. When water is added, the plaster reconstitutes itself into solid gypsum. As a building material, normal plaster alone does not posses the strength needed for load-bearing applications, but gypsum powder can be added to other materials such as Portland cement to produce a lightweight, quick-drying, durable building material.
The use of gypsum plaster dates back to the earliest human civilisations over 9,000 years ago. It was used by the Egyptians, Syrians, Greeks and Romans for building sculptures and monuments and producing casts. Plaster of Paris, a common name for basic white gypsum plaster, got its name from the ancient gypsum quarries near the city of Paris, France. Gypsum cement is still used in building, though it has been largely replaced by Portland cement and other masonry mixes.
Plaster of Paris vs. Gypsum Cement
The term "gypsum cement" generally denotes a variety of gypsum plaster of higher quality or with special properties such as low moisture absorption, high compression strength and fire resistance. Often chemicals or aggregates, such as Portland cement, are added to gypsum cement to achieve these properties. As a result, gypsum cements are generally more expensive than plaster of Paris and require more precision in mixing.
Engineering-grade gypsum cements such as Hydrocal and Hydro-Stone can possess compression strengths of up to 10,000 PSI, making them well-suited to load bearing applications. Gypsum cement, which is much lighter and more fire resistant than mortar cement, is often used as an underlayment for bathroom and kitchen floors, or as cladding and decoration for walls and ceilings. In artistic application, gypsum cement is used to produce large-scale sculptures which must durable and water resistant in order to weather the elements. Gypsum cement is also used for castings and precision tooling in industrial manufacturing processes.
The amount of water needed for each different type of gypsum cement will be represented by a consistency number, which indicates the number of pounds of water per 45.4kg. of plaster. You can scale this number up or down depending on the amount of material you are mixing (e.g. 4.54kg. of plaster with a consistency of 40 will require 1.81kg. of water). Weigh out the amount of water and the amount of plaster and mix them both in a large basin. Most gypsum cements set within 15 to 20 minutes, so mix in small batches to avoid wasting your material.
Gypsum cement is easily inhaled in its powered form and over time can cause respiratory problems due to calcium build-up in the lungs. A respirator or dust mask should always be worn when working with powdered gypsum cement, or when sanding, grinding or tooling dry gypsum cement. The reaction that produces solid material from powder and water also releases energy in the form of heat, proportional to the amount of material being used, which can cause severe burns.